Income Taxes Effective Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate	21.00%	21.00%	35.00%
State income tax, net of federal provisions	0.70%	0.90%	(1.90%)
Recognition of unrecognized tax benefit	(12.50%)	0.00%	0.00%
Flow-through repairs deductions	(10.80%)	(10.80%)	(17.30%)
Production tax credits	(6.30%)	(6.10%)	(6.30%)
Plant and depreciation of flow through items	(2.20%)	(1.20%)	(1.30%)
Amortization of excess DIT	(0.90%)	(2.10%)	0.00%
Impact of Tax Cuts and Jobs Act	(0.10%)	(11.10%)	0.00%
Prior year permanent return to accrual adjustments	0.30%	(1.70%)	(0.30%)
Other, net	(0.10%)	0.60%	(0.30%)
Effective tax rate	(10.90%)	(10.50%)	7.60%
Income Before Income Taxes	$ 182,195	$ 178,250	$ 176,071
Income tax calculated at federal statutory rate	38,261	37,433	61,625
State income, net of federal provisions	1,251	1,613	(3,258)
Recognition of unrecognized tax benefit	(22,825)	0	0
Flow-through repairs deductions	(19,706)	(19,323)	(30,490)
Production tax credits	(11,483)	(10,890)	(11,032)
Plant and depreciation of flow through items	(3,952)	(2,175)	(2,208)
Amortization of excess DIT	(1,688)	(3,731)	0
Impact of Tax Cuts and Jobs Act	(198)	(19,840)	0
Prior year permanent return to accrual adjustments	559	(2,978)	(629)
Other, net	(144)	1,181	(640)
Total reconciling items	(58,186)	(56,143)	(48,257)
Income Tax (Benefit) Expense	$ (19,925)	$ (18,710)	$ 13,368